The present invention relates to a color cathode-ray tube equipped with a shadow mask, and an elastic support member and an elastic support mechanism for the color cathode-ray tube.
In general, a color cathode-ray tube is provided with an envelope that includes a rectangular face panel having side walls on a peripheral edge portion of an effective section thereof, and a funnel coupled to the side walls of the panel. A phosphor screen comprising three color phosphor layers which are able to emit blue, green and red light is formed on the inner surface of the effective section of the face panel. In the envelope, a rectangular shadow mask is opposed to the inside of the face panel. Further, an electron gun for emitting three electron beams is disposed in the neck of the funnel.
The electron beams emitted from the electron gun are deflected by a deflection device mounted on the outside of the funnel, and horizontally and vertically scan the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask is used to sort out the three electron beams from the electron gun and then correctly land them onto the corresponding three color phosphor layers, in order to obtain desired colors. The shadow mask has a substantially rectangular shadow mask body with multiple electron beam passage apertures, and a substantially rectangular mask frame attached to the periphery of the shadow mask body. The mask frame has four side walls. At least three side walls of the mask frame are supported on the corresponding side walls of the face panel by means of elastic support members, so that the mask body opposes to the phosphor screen with a predetermined distance. Each support member has an end portion fixed to the mask frame, and the other end portion engaged with a stud pin which is provided on the inner surface of a corresponding side wall of the face panel.
In the color cathode ray tube as described above, about 80% of the electron beams strike upon the shadow mask. As a result of the striking of the electron beams, the shadow mask is heated and thermally expanded, thereby causing misalignment between the electron beam passage apertures and the phosphor layers. Accordingly, the electron beams having passed through the shadow mask cannot strike on or land on phosphor layers of target colors, with the result that a color image formed on the phosphor screen is degraded in color purity.
To avoid this, a conventional highly fine color cathode-ray tube for a display monitor, for example, uses, as the material of the shadow mask, Invar of a low thermal expansion property which suppresses thermal expansion.
However, it is difficult in light of cost to use expensive Invar as the material of the mask frame as well as the mask body, and therefore iron is usually used for the mask frame. Therefore, when the heat of the mask body has been transmitted to the mask frame, the mask frame made of iron thermally expands much more than the mask body, whereby a peripheral portion of the mask body is pulled by the mask frame, and the electron beam passage apertures are shifted in a radially outward direction with respect to the center of the mask body. Accordingly, the landing position of a beam spot formed by an electron beam, having passed through each electron beam passage aperture is also shifted in a radially outward direction from a corresponding target phosphor layer.
To reduce such a landing shift, the shadow mask is shifted toward the phosphor screen (this shift will hereinafter be referred to as a xe2x80x9cZMF displacement (Z-directional Mask Frame displacement)xe2x80x9d, wherein the Z-direction is defined as the direction of the tube axis), thereby shifting the landing position of the electron beams on the phosphor layer toward the center of the phosphor screen so as to offset the landing position shift due to the thermal expansion of the mask frame. As a result, an electron beam of an appropriate spot size can be landed on its target phosphor layer, and hence color purity degradation can be suppressed.
U.S. Pat. No. 3,803,436 discloses a method for suppressing color purity degradation as above, used in a color image cathode ray tube, and wherein elastic support members which secure the mask frame to the panel are appropriately shaped.
Specifically, each of the elastic support members is formed by bending a substantially rectangular metal plate, and comprises a fixed portion fixed to the mask frame, an engagement portion having an engagement hole to be engaged with a stud pin projecting from the face panel, and an slope portion extending between the fixed portion and the engagement portion. The metal plate is bent along a first bending line located between the fixed portion and the slope portion, and also along a second bending line located between the slope portion and the engagement portion. The first and second bending lines extend at a predetermined angle to the direction perpendicular to the tube axis of the color image picture tube.
Where such elastic support members are used, when the heat of the mask body has been transmitted to the mask frame, and the mask frame has been thermally expanded, the elastic members are each pressed against the side walls of the panel by the side walls of the mask frame. As a result, each elastic support member is elastically deformed in a direction in which the bent portions are opened, thereby causing the mask frame to be displaced toward the phosphor screen. In accordance with this displacement, the mask body secured to the mask frame moves toward the phosphor screen. This makes the landing position of each electron beam shift toward the center of the phosphor screen, whereby the color purity degradation is reduced.
The ZMF displacement of the elastic support member, which has two bent portions inclined to the tube axis, increases as the inclination angle of the bent portions to the tube axis increases. However, the ZMF displacement will not increase after the inclination angle exceeds about 40xc2x0.
In the case of a small-size color cathode-ray tube of about less than 15 inches, color purity degradation due to the thermal expansion of the mask frame can be suppressed using the ZMF displacement obtained when the inclination angle of the bent portions is set at less than 30xc2x0. On the other hand, in the case of a larger color cathode-ray tube of 15 inches, 17 inches or more than 17 inches, a ZMF displacement sufficient to suppress color purity degradation cannot be obtained.
The present invention has been developed to solve the above-described problems, and its object is to provide a color cathode-ray tube in which color purity deterioration due to thermal expansion of a mask frame employed therein can be compensated even when the tube has a large size of 15 inches or more, and also to provide an elastic support member and an elastic support mechanism employed in the color cathode-ray tube for the mentioned purpose.
According to an aspect of the invention, there is provided a color cathode-ray tube comprising:
a panel having a substantially rectangular effective section, and four side walls provided on a peripheral edge portion of the effective section;
a phosphor screen formed on an inner surface of the effective section of the panel;
a shadow mask arranged inside the panel, having a substantially rectangular mask body opposed to the phosphor screen, and also having a substantially rectangular mask frame supporting a peripheral edge portion of the mask body and opposed to the side walls;
a plurality of elastic support members provided between the mask frame and the side walls of the panel and elastically supporting the mask frame on the side walls of the panel, the elastic support members being adapted to displace the mask frame toward the phosphor screen along a tube axis of the color cathode-ray tube when the mask frame thermally expands toward the side walls of the panel; and
an electron gun for emitting electron beams toward the phosphor screen through the shadow mask;
wherein
each of the elastic support members is formed by bending a substantially rectangular plate along first and second bending lines which extend parallel to each other and inclined to the tube axis, and has a fixed portion fixed to the mask frame and being adjacent to the first bending line, an engagement portion engaged with a corresponding one of the side walls and being adjacent to the second bending line, and a slope portion connecting the engagement portion to the fixed portion, and
the slope portion has a responsiveness to a displacement in the mask frame due to thermal expansion of the mask frame, the part of the slope portion, which is close to the first bending line, having a higher responsiveness than the part of the slope portion, which is close to the second bending line.
Preferably, the cross section of each elastic support member, which is in parallel to the first bending line, is smaller at its fixed-portion side than at its engagement-portion side.
In the color cathode-ray tube constructed as described above, the responsiveness of the slope portion of each elastic support member to a displacement in the mask frame due to thermal expansion thereof caused by the heating of the shadow mask is smaller at its engagement-portion side than at its fixed-portion side. Accordingly, when each elastic support member is compressed by the expanded mask frame, the fixed-portion side of the slope portion more greatly deforms than the engagement-portion side thereof, thereby displacing the shadow mask toward the phosphor screen. As a result, the displacement of the shadow mask relative to the phosphor screen, caused by the thermal expansion of the mask frame, can be corrected, thereby compensating the color purity degradation of the image displayed on the phosphor screen.
Such a responsiveness-to-displacement as above can be imparted to each elastic support member when each elastic support member is formed such that the cross section of the slope portion which is parallel to the bending line of a bending portion is smaller at its fixed-portion side than at its engagement-portion side. Further, the responsiveness to displacement can be also obtained by setting longer the one of two bending portions defining the slope portion, which is close to the engagement portion, than the other of the two bending portions, which is close to the fixed portion.
According to another aspect of the invention, there is provided an elastic support member incorporated in a color cathode-ray tube which includes a panel having a substantially rectangular effective section, and four side walls provided on a peripheral edge portion of the effective section, a phosphor screen formed on an inner surface of the effective section of the panel, a shadow mask arranged inside the panel, having a substantially rectangular mask body opposed to the phosphor screen, and also having a substantially rectangular mask frame supporting a peripheral edge portion of the mask body and opposed to the side walls, the elastic support member elastically supporting the shadow mask on the side walls of the panel, comprising:
a fixed portion fixed to the mask frame; an engagement portion engaged with a corresponding one of the side walls; and a slope portion having opposite bent portions which are inclined to a tube axis of the color cathode-ray tube and connecting the fixed portion and the engagement portion to each other with a step therebetween;
the slope portion having a responsiveness to a displacement in the mask frame due to thermal expansion of the mask frame, the responsiveness being higher at a fixed-portion side of the slope portion than at an engagement-portion side thereof.
Preferably, the cross section of the slope portion of each elastic support member, which is parallel to the bent portions, is smaller at its fixed-portion side than at its engagement-portion side.
When the elastic support members, constructed as above, for use in the color cathode-ray tube have been compressed by the mask frame as a result of its thermal expansion, the part of the slope portion, which is close to the fixed portion, is more greatly displaced than the part thereof close to the engagement portion by virtue of the responsiveness-to-displacement of the slope portion. This corrects relative displacement of the shadow mask to the phosphor screen due to the thermal expansion of the mask frame, thereby compensating color purity deterioration of the phosphor screen.
Moreover, in each elastic support member according to the invention, the fixed-portion has a projection projecting from one of the bending portion which is close to the fixed portion, along the axis of the color cathode-ray tube. This projection facilitates the fixing of the fixed portion to the mask frame.
According to a yet another aspect of the invention, there is provided an elastic support mechanism incorporated in a color cathode-ray tube which includes a panel having a substantially rectangular effective section, and four side walls provided on a peripheral edge portion of the effective section, a phosphor screen formed on an inner surface of the effective section of the panel, a shadow mask arranged inside the panel, having a substantially rectangular mask body opposed to the phosphor screen, and also having a substantially rectangular mask frame supporting a peripheral edge portion of the mask body and opposed to the side walls, the elastic support mechanism elastically supporting the shadow mask on the side walls of the panel, comprising:
a stud pin provided on an inner surface of each of at least three of the side walls; and an elastic support member provided between each of the at least three of the side walls and the mask frame.
Each elastic support member includes a fixed portion fixed to the mask frame; an engagement portion engaged with a corresponding one of the side walls; and a slope portion having opposite bent portions which are inclined to a tube axis of the color cathode-ray tube and connecting the fixed portion and the engagement portion to each other with a step therebetween.
The slope portion has a responsiveness to a displacement in the mask frame due to thermal expansion of the mask frame, the responsiveness being higher at a fixed-portion side of the slope portion than at an engagement-portion side thereof.